title: Universal features in the photoemission spectroscopy of high-temperature superconductors.
abstract: The energy gap for electronic excitations is one of the most important characteristics of the superconducting state, as it directly reflects the pairing of electrons. In the copper-oxide high-temperature superconductors (HTSCs), a strongly anisotropic energy gap, which vanishes along high-symmetry directions, is a clear manifestation of the d-wave symmetry of the pairing. There is, however, a dramatic change in the form of the gap anisotropy with reduced carrier concentration (underdoping). Although the vanishing of the gap along the diagonal to the square Cu-O bond directions is robust, the doping dependence of the large gap along the Cu-O directions suggests that its origin might be different from pairing. It is thus tempting to associate the large gap with a second-order parameter distinct from superconductivity. We use angle-resolved photoemission spectroscopy to show that the two-gap behavior and the destruction of well-defined electronic excitations are not universal features of HTSCs, and depend sensitively on how the underdoped materials are prepared. Depending on cation substitution, underdoped samples either show two-gap behavior or not. In contrast, many other characteristics of HTSCs, such as the dome-like dependence of on doping, long-lived excitations along the diagonals to the Cu-O bonds, and an energy gap at the Brillouin zone boundary that decreases monotonically with doping while persisting above (the pseudogap), are present in all samples, irrespective of whether they exhibit two-gap behavior or not. Our results imply that universal aspects of high- superconductivity are relatively insensitive to differences in the electronic states along the Cu-O bond directions.
authors.affiliation: Department of Physics, University of Illinois at Chicago, Chicago, IL 60607.
authors.forename: Junjing
authors.initials: J
authors.lastname: Zhao
authors.forename: Utpal
authors.initials: U
authors.lastname: Chatterjee
authors.forename: Dingfei
authors.initials: D
authors.lastname: Ai
authors.forename: David G
authors.initials: DG
authors.lastname: Hinks
authors.forename: Hong
authors.initials: H
authors.lastname: Zheng
authors.forename: G D
authors.initials: GD
authors.lastname: Gu
authors.forename: John-Paul
authors.initials: JP
authors.lastname: Castellan
authors.forename: Stephan
authors.initials: S
authors.lastname: Rosenkranz
authors.forename: Helmut
authors.initials: H
authors.lastname: Claus
authors.forename: Michael R
authors.initials: MR
authors.lastname: Norman
authors.forename: Mohit
authors.initials: M
authors.lastname: Randeria
authors.forename: Juan Carlos
authors.initials: JC
authors.lastname: Campuzano
mesh_terms: D016880:Anisotropy; D003300:Copper / Q000737:chemistry*; D004553:Electric Conductivity; D008956:Models, Chemical; D056951:Photoelectron Spectroscopy / Q000379:methods*; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D003300:Copper; C030973:cupric oxide